Aerogels are a special class of open-cell foams derived from highly cross-linked inorganic or organic gels that are dried using special techniques to preserve the tenuous solid network. These materials have ultra fine cell/pore sizes, continuous porosity, high surface area, and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 Å. This microstructure is responsible for the unusual optical, acoustical, thermal, and mechanical properties of aerogels. These materials are prepared through the sol-gel process and can be either granular or monolithic.
Organic aerogels are typically prepared from the sol-gel polymerization of resorcinol and formaldehyde and are dried through supercritical extraction of the reaction solvent. Recent efforts have focused on the ability to tailor the bulk properties of organic aerogels for specific applications. One area of interest is the design of carbon aerogels containing different dopants, such as metal ions. For example, a recent article in Advanced Materials 2000, 12, No. 21, November 2 by Bekyarova and Kaneko shows that Carbon aerogels with controlled porosity can be obtained by doping the resorcinol-formaldehyde reaction mixture with Ce and/or Zr by adding Ce(NO3)3 or ZrO(NO3)2 to an aqueous solution of resorcinol and formaldehyde. These researchers found that “not only the initial pH of the solution, but also the nature of the dopant metal, affects the sol-gel chemistry and thus the structure [of the resulting carbon aerogels]. The surface area of the Ce,Zr-doped carbon aerogels ranges from 500 to 800 m2g−1, the micropore volume is between 0.17 and 0.20 m3g−1, and the micropore size is 0.7 nm.”
Other researchers have synthesized carbon aerogels containing transition metals for the purpose of catalyzing graphitization reactions producing unique carbon structures. For example, Maldonado-Hodar et al in a recent article in Langmuir 2000, 16, 4367-4373, describe a method to synthesize Cr-, Fe-, Co-, and Ni-containing carbon aerogels by dissolving resorcinol and formaldehyde in water and adding either chromium nitrate, iron acetate, cobalt acetate or nickel acetate to the solution. The metal content of these carbon aerogels ranged from 1.4 to 5.4%. See Maldonado-Hodar et al, pg. 4368, Table 2.
Metal-doped carbon aerogels have potential technical applications in capacitors, batteries, catalysts and adsorbants.